1. Field of the Invention
This invention relates generally to protecting components of a motor vehicle's all-wheel-drive (AWD) system, particularly its differential and halfshafts, when operating with its disconnect clutch de-engergized.
2. Description of the Prior Art
When an AWD drivetrain operates in disconnect mode, the hypoid ring and pinion gears and one of the output shafts of the Rear Drive Unit (RDU) are disconnected from the drivetrain by low-drag torque clutches, thereby preventing transmission of rotary motion and torque from the engine and vehicle wheels to the hypoid ring and pinion gears and driveshaft. A first RDU output shaft, which is connected to its respective differential side gear, rotates at a speed corresponding to vehicle speed. A second RDU output shaft, which is disconnected from the drivetrain, rotates at the same speed as that of the first output shaft but in the opposite direction. The relative speed across the differential assembly, which is referred to as differential speed, is two times the rotational speed of the first output shaft.
A purpose of an AWD disconnect system is to minimize parasitic losses of the AWD system when not engaged, thus significantly improving fuel economy during most driving conditions. When in AWD disconnect mode, the differential speed at maximum vehicle speed can be extremely high. If the hypoid ring and pinion gears are stationary, the differential housing does not rotate. Therefore, the differential pinions contained within the differential housing, which rotate at high differential speeds, can remain above the sump oil level for an extended period of time. This condition reduces ability to reject heat.
The previous solutions involved the addition of special surface treatments, extra parts such as bearings, and more elaborate lubrication methods, all of which increase the cost of the RDU. A need exists for a technique to prevent damage to the RDU differential and output shafts due to inadequate heat rejection at the interfaces experiencing high relative speed while in AWD disconnect mode.